A Collection is an Object that has items or orderedItems

however, orderedItems is valid on both Collection and OrderedCollection! it is defined like this in the as2 context:

"orderedItems": {
	"@id": "as:items",
	"@type": "@id",
	"@container": "@list"
}

so it is basically just an alias for items where it MUST be an array, and the array’s order matters. (if it were @container: set, then it MUST be an array, but the array’s order does not matter.)

tangent: a Collection may be ordered without being an OrderedCollection [link]

OrderedCollection is defined as strictly reverse chronological by ActivityPub. however, other orderings are valid on regular Collections. the use of the orderedItems term allows plain-JSON implementations to do exactly this.